Looking back to the history to jump ahead in the future.
The Synchronous Reluctance (SR) motors driven by variable speed inverters have been recently introduced in the Industrial market. They are capable to achieve very high efficiency level and to gain the maximum efficiency class (IE4) of IEC 60034-30 standard. Due to the wide use of electric motors in the world – take into account that electric motors consume the 70% of the electricity in EU industries – the improvement of their energy efficiency has become a major research focus. Both energy consumption as well as the environmental impact can be significantly reduced by adopting electric motors with high efficiency levels.
The SR motors answer to such request of high efficiency at very high performance/cost ratio, being the right solution not only for Industrial market but also for Material Handling and Electrical Mobility markets: their technical characteristics hugely improve the ones of ACIM motors at the same cost level.
The invention of the SR motor concept dates back to 1923  and its theoretical analysis have been widely investigated in the subsequent years and in the 2nd half of 1900 thanks to the work of Alger , Lawrenson , Cruickshank  , Honsinger , Miller  and Staton  among others, but only with the more recent improvements of the rotor design, the advance of power converters and related control algorithms, this drive technology has become suitable for industrial use. Extended analysis of synchronous relactance machines, their control algorithms, related examples and experimental results can be found in  to .
The relatively high cost of concurrent motor technologies (Internal Permanent Magnet motors – IPMm – and Permanent Magnet Superficially Mounted motors – PMSMm) and the technical advantages over ACIM ones make the SR motors serious candidates as winning technology in many future industrial markets.
In order to understand the working principle of such motor, you should realize that the SR rotor has neither a conducting short circuit cage—as the IM does—nor permanent magnets or field excitation winding, but the SR machine utilizes the reluctance concept and rotating sinusoidal MMF, which can be produced by the traditional IM stator, for torque production.
The complex and anisotropic design of the rotor lamination generates different (geometric) reluctances in the d-axis and the q-axis and, if a magnetic field is applied to such anisotropic rotor, torque is produced as the rotor attempts to align the magnetically conducting direction to the stator field.
In the SRM, the stator field is produced by a sinusoidally distributed winding in a slotted stator and it links the stator and rotor through a small air gap, exactly as in a traditional ACIM. The field is rotating at synchronous speed and can be assumed to have a sinusoidal distribution. Basically the synchronous reluctance motor is a true AC rotating field machine, requiring a balanced polyphase sinusoidal supply into a distributed winding, which for all intents and purposes is identical to that of the induction machine of the same power rating.
SR motor principle and related torque general equation:
Basic design of the SR motor:
The characteristics of such design are:
The inherent advantages of SR motor technology over IM technology are:
Just to focus on further considerations about power losses, in modern IMs with a short circuit rotor cage, the losses associated with the rotor amount to 20–35 percent of the total motor losses. Synchronous rotation eliminates most of these associated losses.
The elimination of these slip losses leads to an efficiency increase of 4 to 8 percent, as well as a 20 to 40 (depending on motor power) percent increase in power and torque density for the same insulation temperature class.
For a complete analysis, the characteristics that could be considered disadvantages are:
Thanks to such characteristics, the SynRM can be designed for the same frame size as an equivalent power IM, but achieving the latest IE4 efficiency standard rating. You have to take into account that the majority of the losses in the synchronous reluctance machine are confined to the stator copper windings, where it is generally easier to remove heat than the rotor, which is a major challenge in the Induction Machines.
The SR motor has a rocky history in its development but has had a fraction of the attention and expenditure deserved to different motor technologies (ACIM and PM in particular).
SME S.p.A. has strongly believed that the inherent advantages of SR motors could provide improved engineering solutions for many applications and has invested in 4 years of research to analyze, develop and optimize such technology.
The development of patented design of rotor lamination together with the implementation of rock solid, state-of-the-art control algorithms let us say that the SR motor technology developed by SME rivals with ACIM motor technology for power density from 0 speed to 3 times nominal/based speed while it’s widely better under efficiency and thermal points of view.
If you consider the SR motor increased robustness and lower maintenance and cost, its magnet-free design, the relative low cost impact of the investments for its mass production since it’s a true AC machine as IM motors already in the market, you can realize how easy and effective can be the replacement of ACIM motors in many applications and how it can compete with PM motors in many others, owing to their losing use of expensive magnets.
SME has engineered and manufactured SR motors in a wide range of output power, suitable and available for applications in the industrial, material handling and automotive markets and will continue working on their improvement and enlarging their power range since such arising technology will be the next big jump ahead in the future for a wide range of electrical motor applications.
References Kotsko, “Polyphase Reaction Synchronous Motor”, Journal of AIEE, vol.42 pp.1162-1168, 1923  P. L. Alger, “The Nature of Polyphase Induction Machines”, New York: Wiley, 1951.  Lawrenson, P.J., Agu, L.A., “Theory and performance of polyphase reluctance machines”, Electrical Engineers, Proceedings of the Institution of, vol.111 no.8 pp.1435-1445, August 196  Cruickshank A.J.O. et al., “Theory and performance of polyphase reluctance motors with axially laminated anisotropic rotors”, Proc. IEE, vol.118 no.7, 1971  Honsinger V.B., “The inductances Ld and Lq of reluctance machines”, IEEE Trans. PAS, vol.90 no.1, 1971  Miller, T.J.E., Cossar, C., Hutton, A.J., “Design of a synchronous reluctance motor drive”, Industry Applications Society Annual Meeting, 1989, Conference Record of 1989 IEEE, vol.1 no. pp.122-127, 1-5 October 1989  Staton D.A., Soong, W.L.; Miller, T.J.E., “Unified theory of torque production in switched reluctance and synchronous motors”, Industry Applications, IEEE Transactions on, vol.31 no.2 pp.329-337, March/April 1995  A. Vagati, G. Franceschini, I. Marongiu, G.P. Troglia, “Design criteria of high performance synchronous reluctance motors”, Industry Applications Society Annual Meeting, 1992., Conference Record of the 1992 IEEE 4-9 Oct. 1992 Page(s):66 – 73 vol.1.  A. Fratta, A. Vagati, F. Villata, “On the evolution of A.C. machines for spindle drive applications”, Industry Applications, IEEE Transactions on, Volume 28, Issue 5, Sept.-Oct. 1992 Page(s):1081 – 1086.  A. Vagati, A. Fratta, G. Franceschini, P. Rosso, “AC motors for high-performance drives:a design-based comparison”, Industry Applications, IEEE Transactions on, Volume 32,Issue 5, Sept.-Oct. 1996 Page(s):1211 – 1219.  J.J. Germishuizen, F.S. Van der Merwe, K. Van der Westhuizen, M.J. Kamper, “Performance comparison of reluctance synchronous and induction traction drives for electrical multiple units”, Industry Applications Conference, 2000. Conference Record ofthe 2000 IEEE, Volume 1, 8-12 Oct. 2000 Page(s):316 – 323 vol.1.  F. Parasiliti, M. Villani, “Synchronous reluctance motors design and performance prediction using an analytical procedure”, Electrical Engineering and Electromagnetics, WIT Press, 2003, ISBN- 1-85312- 981-X, pp. 147-156.  F. Parasiliti, M. Tursini, M. Villani, “Design of synchronous reluctance motors by finite element analysis and optimization algorithm Prototype and tests”, Department of Electrical and Information Engineering, University of ‘Aquila, 67100 Poggio di Roio,L’Aquila, Italy, July 15th 2006.  Rae Hwa Lee, Min Myung Lee, Jung Ho Lee, “Rotor design functional standard of Synchronous Reluctance Motor according to torque per volume using FEM & SUMT”,Manuscript received June 30, 2006.2  T. Matsuo, T. A. Lipo, “Field oriented control of synchronous reluctance machine”, Power Electronics Specialists Conference, 1993. PESC ’93 Record, 24th Annual IEEE, 20-24 June 1993 Page(s):425 – 431.  A. Boglietti, A. Cavagnino, M. Pastorelli, A. Vagati, “Experimental comparison of induction and synchronous reluctance motors performance”, Industry Applications Conference, 2005. Fourtieth IAS Annual Meeting, Conference Record of the 2005,Volume 1, 2-6 Oct. 2005 Page(s):474 – 479 Vol. 1.
SME NA Inc.
1404 King St N, Box 154,
St Jacobs, Ontario, N0B 2N0, Canada
PHONE +1 519 591-0965